New field observations and U-Pb ages in the Sudbury area: toward a detailed cross-section through the deformed Sudbury Structure

The Sudbury area straddles the transition from the Archean Superior structural province to the Paleoproterozoic Southern province. To the south it is flanked by younger Proterozoic belts and finally the ca. 1 Ga Grenville Front. It is unique in that it also hosts the deformed remnants of one of the largest and oldest preserved impact structures in the geological record, the ~300 km diameter Sudbury Structure. This structure is characterized by a differentiated melt sheet, commonly referred to as the Sudbury Igneous Complex (SIC), which at or near its base hosts one of the largest concentrations of Ni-Cu-PGE sulphides on the planet. These metal-rich sulphides have formed the basis for an extensive mining industry since nickel was first discovered during railway construction in 1883. Despite more than a century of research, many geological questions remain unresolved in this fascinating area. Here we present new field observations and preliminary U-Pb zircon and baddeleyite ID-TIMS results on a suite of about 20 critical samples that help resolve some long-standing geological questions. Many rock units in the Sudbury area have experienced significant shock metamorphism, which has increased the complexity of the Pb-loss patterns of their zircon crystals. Therefore, a key rationale for the present study was to apply "chemical abrasion" pre-treatment to single best-preserved zircon crystals, or fragments thereof, to reduce or eliminate young Pb loss and allow us to see through the shock-induced Pb loss. We show that the Joe Lake Gabbro below the North Range is an Archean metagabbro, consistent with observed field relationships. Foliated granite on the Southeast Range is also Archean, requiring that metavolcanic rocks it intruded are Archean as well and not part of the basal Huronian rift succession. We present the first robust age on the Creighton Granite, showing it to be a folded subvolcanic sill and the magma chamber to the overlying Copper Cliff Rhyolite. Together, the Creighton Granite and Copper Cliff Rhyolite represent a single felsic magmatic system 24552460 Ma in age, which developed in the immediate aftermath of the main pulse of Matachewan mafic magmatism at ca. 2460 Ma. Magma mingling structures near the base of the Creighton Granite sill demonstrate the intimate relationship with Matachewan mafic magmas. We have dated a number of mafic dyke swarms in the area, both pre- and post-dating the SIC. Among these is the first recognition of a ca. 2507 Ma dyke swarm in the Sudbury area. Furthermore, we present several ages on the SIC and its offset dykes, including a weighted mean 207Pb/206Pb age of 1849.7 ±0.2 Ma for a high-MgO norite in the South Range, and a precise concordant baddeleyite age of 1848.5 ±0.8 Ma for the radial Pele dyke on the North Range. The latter likely represents the youngest and final dyke injection of the offset dyke system into the fractured footwall of the SIC.